In the field of radio communications, various communication systems are being studied and put to practical use aiming at improvement in frequency utilization efficiency. OFDM (Orthogonal Frequency Division Multiplexing) that is one of these systems is robust against multi-path delay waves, and is regarded as favorite as the fourth generation mobile communication system.
A typical radio transmitter to which the OFDM modulation system is applied performs primary modulation on a bit string of transmit data with a modulator of QAM (Quadrature Amplitude Modulation) etc. to convert the transmit data into complex symbols on a predetermined number of bits basis. In the case where a 16 QAM modulator is used as the primary modulator (symbol mapper), the transmit data is converted into a complex symbol sequence of QAM on a four-bit basis.
The complex symbol sequence generated by the symbol mapper is broken into blocks with a predetermined number of symbols by a serial to parallel converter. In the following explanation, the number of complex symbols of a block into which the data is broken by the serial to parallel converter is designated by NSC. NSC complex symbols that are outputted from the serial to parallel converter after having been blocked are added with an arbitrary number of zero-value symbols so that the total number of the complex symbols may become a number that can be expressed by powers of 2 (hereinafter designated by NFFT). The signal block having an increased number of symbols is subjected to secondary modulation by an inverse fast Fourier transform (IFFT) device to be converted to NFFT complex numbers that will be a sampling number of OFDM symbols.
In OFDM, since data is modulated by NSC subcarriers that differ mutually in frequency and is correlated with non-zero complex symbols by the IFFT device among NFFT orthogonal subcarriers, OFDM may be called a kind of multicarrier modulation system.
In OFDM, principally, NFFT complex symbol values (sampled values) outputted serially from the NFFT device are converted into a serial signal sequence by a parallel to serial converter, becoming complex baseband OFDM signals. After being converted into the analog continuous signal by the D/A converter, the complex baseband OFDM signal is multiplied by a carrier wave, being converted into an OFDM signal in the RF waveband. The OFDM signal that became a serial signal and constitutes one block is called an OFDM symbol, and its length is called a symbol length.
Since the OFDM signal transmitted after being power amplified by the RF transmitting unit arrives at a receiver being divided into a direct wave that does not suffers reflection on the way of transmission and a delayed wave that is reflected by obstacles on a transmission path, the delayed wave acts as noise against the direct wave. Then, in order to remove an effect of the delayed wave, the OFDM transmitter forms an OFDM symbol that was made redundant and consists of (NFFT+NCP) sample points by copying NCP sample points belonging to the second half part in each OFDM symbol that consists of NFFT sample points and inserting them into the first half part. The above-mentioned redundant portion is called a cyclic prefix, and acts as a guard interval for removing the effect of the delayed wave. If the cyclic prefix length NCP is longer than or equal to the delay time of the multi-path delay wave, the effect of the delayed wave can be removed on the OFDM receiver side.
The parallel to serial converter is converting the (NFFT+NCP) sample points thus obtained into serial signals. Moreover, since a transmission spectrum of the OFDM signal transmitted from the RF transmitting unit tends to widen because the signals become discontinuous between the symbols, the signals are configured to be continuous by conducting taper processing on a connection part between the symbols using an appropriate window function after inserting the guard interval (cyclic prefix). Moreover, as necessary, a band limiting filter is provided to suppress the widening of the transmission spectrum.
In the OFDM system, multiple subcarriers each having a different frequency are used in modulating the transmission symbol, and so each transmission symbol can be regarded as a mutually decorrelative random signal. Therefore, it is known that according to the central limit theorem, a signal distribution approaches a normal distribution and a peak factor (the ratio between the maximum electric power and the average electric power) of a transmitted wave will also amount to 10 dB-12 dB. Although a modulated signal is transmitted after being amplified by the power amplifier of the RF transmitter, generally, there is a limitation in the linear area of the amplifier and the modulated signal of the OFDM is saturated with a large output. Therefore, when the power amplifier gives rise to saturation to a peak amplitude of the transmitting signal, distortion arises in a transmission waveform, which will leak an electric power to the outside of a transmission wave band, especially to adjacent frequency bands. Since this leakage electric power is strictly regulated by the radio wave regulations, it becomes difficult for the transmitter of the OFDM to increase a transmit power sufficiently up to a rated output of the power amplifier.
In order to solve such a problem, peak factor reduction of the transmitting signal becomes effective. The peak factor reduction here means signal waveform processing such that the peak amplitude is controlled while allowing slight deterioration, namely, addition of noise, in waveform quality. As one of the conventional technologies of the peak factor reduction, for example, JP-A 2003-124824, “PEAK FACTOR REDUCTION DEVICE” (Patent document 1) proposes a peak factor reduction device that, taking a baseband signal of CDMA as an object, generates a correction signal in the shape of impulse at a sample point where the peak of the amplitude becomes maximum, reshapes this signal in waveform with a filter having an equivalent frequency response as that of a baseband band limiting filter, and subtracts it from the baseband signal. Since the correction signal is not a component contained in the original signal, it acts as noise to the transmitting signal. However, since the waveband of the correction signal is masked so as to be in a range of the transmission spectrum of the filter, noise leakage to the outside of the transmission band is prevented.